Switched capacitor filters have made an exciting entry into active filter designs in the past several years by shrinking the size and power required to implement complicated frequency responses. An important part of many communications filter designs involves taking measures to adjust the phase of frequency selective filters to be more linear in nature. This is done so that information at different parts of the passband will reach following circuitry (often a phase sensitive demodulator) at the same time. For narrow bandwidth filters, a number of high Q allpass biquadratic sections are placed at key frequencies to perform the phase equalization. The overall response can be very sensitive to any pole (or zero) location variations in these sections placing great demands on the accuracy of the allpass filter implementation. In monolithic switched capacitor implementations of filters, it is necessary for economical reasons to keep the number of amplifiers in the filter topology and the ratio of the capacitors in the circuit to a minimum. Filters that require huge ratios or many amplifier stages consume too much silicon area to effectively manufacture the chip at a competetive cost.
Employing the generalized two amplifier topology described in the cited references for an allpass filter leads to a capacitor ratio that is "Q" (or Quality Factor) times the integrator capacitor ratios. The sensitivity of the filter characteristics to the capacitors forming this large ratio is such that the capacitor geometries must obey equal area to perimeter matching to maintain accuracy over processing variations.
Two topologies are described in this invention for implementing an allpass biquadratic filter. By making use of a simple summation function (which consumes little overhead area) in the next stage, the largest capacitor ratio in one circuit is reduced to "Q" which matches the ratio requirements of the other filter types when built with the two-amplifier general topology. In the second circuit described, the large ratio exists but it does so only as an artifact of the sampled data domain. Since it only compensates for the effects of switching, and typical clock frequencies employed minimize these effects, the sensitivity to the smallest capacitor used to form the large ratio is very small. This allows the abandoning of area to perimeter matching and the overall silicon area is then the same as a filter with a maximum capacitor ratio of "Q" again.
Additionally, each topology described has a separate output that is not available in the allpass circuits described in prior art. The output exhibits either a bandpass or bandstop characteristic that has the same center frequency and "Q" as the desired allpass filter. This can be extremely useful for other signal processing such as tone detection or rejection.